There is a need for implantable stimulation devices, such as cochlear implants, that can elute pharmacologically active substances following implantation. Such devices could, for example, improve the signal quality of a cochlear implant.
For many patients that are profoundly deaf, their deafness is a result of the loss or absence of hair cells in the cochlea that are necessary to transduce acoustic signals (e.g. sound energy) into auditory nerve impulses. These patients have sensorineural hearing loss (as opposed to conductive hearing loss that can often be treated with conventional hearing aids). Without hair cells, patients with sensorineural hearing loss are unable to generate auditory nerve impulses directly from sounds.
Many cochlear implants have been developed to treat patients with sensorineural hearing loss. These implants are capable of directly stimulating auditory nerve fibers, therein bypassing the hair cells, and producing the perception of sound in the brain and thus some sense of hearing. Such implants normally comprise a set of electrodes (an electrode array) that are implanted into the cochlea. These electrodes are designed to respond to external electrical stimuli, and in turn transmit these impulses to the ganglion cells and the auditory nerve fibers. In an ideal situation, the electronic circuitry combined with the electrodes of the cochlear implant would separate an acoustic signal into narrow bands of frequencies, and these frequency bands would be conveyed selectively to the auditory nerve cells normally responsible for transmitting those frequencies to the brain.
The electrode array of the cochlear implants is generally implanted in the scala tympani, one of three ducts of the spiral shaped cochlea, for best results. The array usually consists of a thin, elongated, flexible carrier containing six to thirty separate electrode contacts, and is inserted by the surgeon into the scala tympani duct. When electrical impulses are delivered from the individual electrodes to tissues and fluids in close proximity, spiral ganglion cells and their auditory nerve fibers create action potentials.
Despite tremendous success in restoring sound sensation to patients that are profoundly deaf with cochlear implants, performance of the implants can often be suboptimal due to (i) apoptosis and/or necrosis of nervous tissue resulting from the trauma of inserting the electrode array, and (ii) a rise in electrode impedance post-surgery that is due to the encapsulation of the electrode array by the growth of a fibrous membrane which reduces the efficiency of electrical stimulation.
Efforts to address these issues include a single intraoperative intracochlear application of corticosteroids during cochlear implantation to reduce impedances at the electrode contacts, and surface patterning of the electrode array to reduce growth of cells on the implant surface (see US Patent Application No. 20060020318 to Lenarz, et al., “Patterning of the surface of implantable devices for tissue growth management”). In addition, U.S. Pat. No. 6,309,410 to Kuzma, et al. describes a cochlear electrode with an incorporated drug delivery channel for application of drugs, and US Patent Application No. 20070213799 to Jolly, et al. describes cochlear implants with material in areas of the implant adapted to elute drug.
There is thus a need to develop materials that can minimize the force required to insert an electrode array into the cochlea, and thereby limit the trauma to the spiral ganglion cells.
There is also a further need to develop technology that allows pharmacologically active substances to be delivered to the inner ear after cochlear implantation, but without surgery. Delivery of pharmacologically active substances in the inner ear after cochlear implantation could (a) provide therapeutic treatment for trauma resulting from insertion of the electrode array, and (b) decrease fibrous growth. In addition, infection could be prevented or treated locally with antibiotics.
It is an object of the present invention to provide polyhydroxyalkanoate (PHA) coatings for implantable stimulation devices, including cochlear implants, wherein the coating provides the electrode with good lubricity to minimize trauma to tissues.
It is a further object of the present invention to provide implantable stimulation devices, including cochlear implants, which can deliver pharmacologically active substances, and comprise PHA polymers and copolymers.
It is another object of the present invention to provide PHA polymer and copolymer drug delivery systems for implantable stimulation devices that can be used to make cochlear implants with excellent physical and mechanical properties and biocompatibility.